This invention generally relates to safety devices for pressurized systems and storage systems typically maintained at or near atmospheric pressure. More particularly, the present invention relates to a pressure relief device for a sealed system.
There are many industries in which it is desirable to design and build systems to hold or process a pressurized fluid. To ensure the safety of these types of systems, each such system typically includes a safety device designed to prevent the over-pressurization of the system. In an emergency situation, such as a fire, where the pressure in the system endangers the physical integrity of the system, the safety device will create an opening to provide a vent path to correct the over-pressurization within the system. The opening allows fluid to vent from the system to the environment, or to a safety reservoir, to reduce the pressure in the system and prevent the system from failing under the pressure of the fluid.
In some industries, such as the food processing industry, it is also expected that systems, which require protection from over-pressure situations, may also be exposed to under-pressure, or vacuum, situations. In certain systems, any vacuum within the system can damage sensitive equipment. In the food & drug processing industries, for example, a vacuum may be created in a system when the system is being cleaned after a completion of a process. Typically, these food processing systems are cleaned and/or sterilized with a steam spray apparatus that removes any product from the system after a processing operation is completed and before the next processing step begins. If the system is not properly controlled during steam cleaning, a sudden vacuum can be developed, which may cause damage to the system. For example, if cold water were introduced to the system while steam cleaning, the steam would condense, thereby creating a vacuum situation.
Thus, to completely protect such a system, the pressure release device must provide two-directional pressure relief. The first direction of pressure relief prevents damage or safety hazards resulting from an over-pressurization, or a positive pressure differential situation. The second direction of pressure relief prevents damage or safety hazards resulting from under-pressurization, or a negative pressure differential situation. Since the design of pressure systems and atmospheric storage systems typically produce a design strength that is much higher in a positive pressure differential than a negative pressure differential, an appropriate two-directional pressure relief device must have the ability to function at pressures that vary significantly in each direction.
It should be noted that some systems are unlikely to encounter an over-pressure situation and, thus, the only risk is exposure to a negative pressure differential. In these types of systems, a pressure relief device need only protect the system from a negative pressure differential.
Some systems require pressure protection at very low levels, measured in "inches of water column" rather than "pounds per square inch." Protection may be required in both over-pressure and under-pressure directions, or just in one direction.
A typical two direction pressure relief device includes a sealing member that is sealingly engaged with the system. The sealing member is surrounded by a pair of support members. One support member is configured to permit the seal to release when the seal is exposed to a predetermined positive pressure differential and the other sealing member is configured to permit the seal to release when the seal is exposed to a predetermined negative pressure differential. To provide protection from a negative pressure differential only, the positive pressure support member may be omitted.
The positive pressure support member provides a backdrop for the sealing member and is configured to withstand a predetermined force. As the pressure in the system rises, the seal moves against the positive pressure support. When the pressure reaches a predetermined level, the positive pressure support releases the seal to create a vent path and reduce the pressure in the system. Typically, the positive pressure support member is a generally solid unit that has a series of holes and slits or perforations. The holes allow fluid to enter the system if the seal releases under a negative pressure differential and the slits allow the support member to open when the positive pressure differential reaches a predetermined level. However, in low pressure applications, the positive pressure support does not always fully open, which results in an obstructed flow path for the venting fluid.
The negative pressure support, often referred to as a "girdle," is typically disposed between the system and the seal. When a light negative pressure differential acts on the seal, the seal moves towards the system and into contact with the girdle. The girdle is configured to flex as the seal is pushed against the girdle by the negative pressure differential. This flexing begins at a predetermined pressure level. In most cases, however, the force of the vacuum on the seal and girdle arrangement will not open the seal. Thus, a knife blade is often positioned to puncture the seal as the girdle and seal flex under the vacuum to ensure that the seal fully opens. The positive pressure support does not open under these conditions, which restricts the flow path for the venting fluid to typically 50% of the pressure relief device nominal cross sectional area.
These types of bidirectional pressure relief devices are not conducive to industries, such as the food & drug industry, that require a sanitary environment for processing operations. The girdle, which is positioned between the seal and the system, tends to pick up product during system operation. The configuration of the systems and the pressure relief devices make it difficult to clean the girdle, especially the space between the seal and the girdle, which may ultimately lead to contamination and/or a spoiled product.
The same girdle and seal combination is typically used to provide a single direction low pressure relief device for either positive or vacuum relief.
It is also desirable to include a sensor in the pressure relief devices to indicate when the seal has opened. Typically, the sensor is integrated with the seal and consists of a wire connection that extends across the seal. In theory, when the seal opens, the wire connection will be broken and will indicate that the seal is open. In operation, however, the seal does not always fully open and the wire connection is not always broken. Thus, the seal may open without activating the sensor. For low pressure relief devices, the sensor may be of such fragile construction to avoid interference with the set pressure of the device, that the sensor wire can break leading to false signals that the seal is open.
In light of the foregoing, there is a need for a pressure relief device that (1) provides a high flow area for both positive and negative pressure releases; (2) includes a reliable sensor to indicate when the seal has opened; (3) provides a sanitary seal, (4) provides a high-flow area for single direction of operation at low pressures, and (5) provides a two-way device that operates at low pressures in both directions or at widely different set pressures in each direction.